There are many different types of heat sources which are employed to bond thermoplastic materials. Impulse heaters and electric resistance heaters are typically employed over a wide range of thermoplastic materials to form bonds. One type of heating has limited use, radio frequency or rf heating. Electromagnetic energy at radio frequencies is used to efficiently heat certain materials. However it is limited to heating those materials which are referred to as dielectrics. A dielectric material is one in which it is possible to store electrical energy by the application of an electric field. The energy is recoverable when the field is removed. Dielectric heating is the result of the interaction of the electromagnetic energy with various components in the atomic or molecular structure of the dielectric. An alternating electrical field causes oscillatory displacements in the charged components of the dielectric.
The chemical structure of a material determines to a large extent the dielectric nature of a material (typically defined in terms of the dielectric constant). The dielectric constant of a material is defined as the ratio of the capacitance of a material in a given electrode configuration to the capacitance of the same electrode configuration with a vacuum as the dielectric. Its value for any material decreases with increasing frequency showing decreasing response to the electric field. A material absorbs energy at a rate given by the equation P=0.555 fE.sup.2 e' tan d X 10.sup.-6. In this equation tan d (d is delta) is called a loss tangent or dissipation factor and indicates the fraction of the stored energy which is converted into heat by the dielectric. P=the heat generated in watts which is the dielectric loss; f is the frequency in MHz; E=the field strength in V/cm and e' is the dielectric constant. The loss index is the product of the dielectric constant of a material and the loss tangent.
The ease with which any material can be dielectrically heated is thus determined by its dielectric constant and its loss tangent. Materials which have relatively good response to dielectric heating include melamine-formaldehyde resins, phenol formaldehyde resins, polyurethanes, polyvinyl chloride, and urea-formaldehyde resins. Materials which have relatively little response to dielectric heating include the silicones, polytetrafluoroethylene, polystyrene, polypropylene and polyethylene which is particularly difficult to heat dielectrically.
Dielectric heating has been most useful in welding thermoplastic materials which have appropriate dielectric constants and loss tangents. Typically two pieces of thermoplastic material are sealed together by positioning them between a metallic electrode and a steel bed plate of a pneumatically operated press. The electrode is pressed against the thermoplastic material as high frequency voltage is applied causing the plastic to heat and melt. The metallic electrode and steel bed do not get hot under the influence of the high frequency energy and in fact draw heat from the material being bonded.
When the plastic is melted, power is turned off and the cool metal electrode and bed rapidly refreeze the plastic. Refreezing under pressure provides a good bond. Further, the outer surfaces of the film can be kept below the melting point and thus maintain their original characteristics.
Vinyl film is most frequently bonded using this method. This method per se is unsuitable for bonding a material having a low loss index. Accordingly, methods have been developed whereby materials with such low factors can be bonded. Particularly these methods for radio frequency bonding of low loss plastics are disclosed in several U.S. patents and these include:
Collins U.S. Pat. No. 2,570,921 PA1 Zoubek U.S. Pat. No. 2,667,437 PA1 Drittenbass U.S. Pat. No. 3,126,307 PA1 Reesen U.S. Pat. No. 3,232,810 PA1 Peterson U.S. Pat. No. 4,268,338.
For example the Collins reference discloses a method of bonding thermoplastic material such as polyethylene using high frequency radio waves. Collins suggests that a wide range of different materials which have a high power factor (roughly equivalent to their loss index) including polyvinyl compounds can be placed above and below materials which have a low power factor or loss tangent and then bonding or welding of these materials using rf energy.
The Zoubek patent discloses placing a buffer material between radio frequency generating electrodes and the material being bonded. Zoubek indicates that the buffer material must have a fairly high dielectric loss index, high thermal conductivity and a high softening point relative to the material to be bonded. The only material specified by Zoubek as an appropriate buffer was nylon. Nylon has a low loss index at 27 MHz but higher at higher frequencies.
Drittenbass discloses the use of polyvinyl chloride as a buffer on one side of two plys of a thermoplastic material to be radio frequency welded. Drittenbass specifically discloses the use of a polyvinyl chloride impregnated sheet.
These patents generally disclose higher frequency equipment (but see Drittenbass). High frequency equipment generally requires the use of a small electrode to provide even heating or power distribution across the entire electrode. This is not practical for high frequency equipment which employs a long bar electrode having a length substantially longer than 12 inches, generally 36' or more in length.
Over the years the radio frequency bonding equipment has been modified. The original equipment typically operated at 12 to 60 and some equipment operates as high as several hundred MHz. Current equipment is generally limited to 27.12 .+-.0.26 MHz. This in effect has caused a reduction in the amount of energy absorbed by a dielectric. Although the field strength has been increased somewhat to compensate, this current equipment does not heat dielectrics as readily as equipment has in the past. In fact it is extremely difficult to use the buffers disclosed in many of the references to dielectrically bond even polyethylene which has a relatively low melting point. Generally efore any bond occurs, there is breakdown which basically burns a hole through the polyethylene.